Lets assume there is $100$ units of code to be executed and let $1$ unit of code takes $1$ second to execute.
Using $4$ processors
$10$ units are executed in $10$ seconds
$90$ units are divided among $4$ processors and executed parallely so the time it takes $= 90/4 = 22.5\approx 23$ seconds.
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$Effeciency = \frac{Boxes \ used}{Total \ Boxes}=\frac{100}{132} = 0.757$
Using $8$ processors
$10$ units are executed in $10$ seconds
$90$ units are divided among $8$ processors and executed parallely so the time it takes $= 90/8=11.25 \approx 12$ seconds.
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$Effeciency = \frac{Boxes \ used}{Total \ Boxes}=\frac{100}{176} = 0.568$
$\therefore$ Option $C.$ is the most appropriate choice.